Added option for square thumbnails

git-svn-id: https://plugins.svn.wordpress.org/flowgallery/trunk@898896 b8457f37-d9ea-0310-8a92-e5e31aec5664
Added option for square thumbnails

git-svn-id: https://plugins.svn.wordpress.org/flowgallery/trunk@898897 b8457f37-d9ea-0310-8a92-e5e31aec5664